1. Field of the Invention
The present invention relates to an electro-photographic image printing apparatus such as a copying machine, printer, or facsimile machine and, more particularly, to a fixing section employed in the image printing apparatus.
2. Description of the Related Art
Generally, in an electrophotographic image printing apparatus, a charging means, exposing means, and developing means are arranged around an image carrier. Charging, exposure, and development are performed to form a toner image on the image carrier. The toner image is transferred onto a recording medium to form an unfixed toner image. A fixing means is widely used in which the unfixed toner image on the recording medium is sandwiched and conveyed by a heat roller and a pressure roller in tight contact with the heat roller so as to fix the toner image.
In such a fixing section, when the length in the convey direction of the recording medium is larger than the circumferential length of the heat roller, after the leading edge in the convey direction of the recording medium comes into contact with the heat roller, the recording medium is sandwiched and conveyed by the rotating heat roller and rotating pressure roller. The heat roller that rotates for the second turn comes into contact again with the trailing edge of the recording medium which is still in contact with the heat roller (which has not passed through the nip portion yet). This trailing edge of the recording medium is called an overlapping portion.
The heat roller at the overlapping portion is deprived of heat while it is in contact with the recording medium, and the heat roller temperature accordingly becomes lower than a normal fixing temperature. This causes problems such as so-called under-fixing in which fixing is not performed completely and so-called reverse surface soiling in which the unfixed toner on the heat roller is transferred to the pressure roller to soil the reverse surface of the recording medium. Even if such an extreme inconvenience does not occur, the gloss of the overlapping portion becomes lower than that of a leading portion of the overlapping portion to form a clear boundary between the overlapping and leading portions that appears as a difference in gloss to degrade the image quality.
In order to solve the above problems, a method is proposed in which the circumferential length of the heat roller is set be equal to the sum of the length of the recording medium and the interval length between the current and next recording media, or an integer multiple of the sum (for example, see Japanese Unexamined Patent Publication No. 8-146797 as patent reference 1).
Another method is also proposed in which the circumferential length of the fixing member is set to be equal to or larger than the length in the convey direction of a standard size recording medium which is used most frequently, or equal to or larger than the length of the short side of the standard size recording medium (for example, see Japanese Unexamined Patent Publication No. 2002-49264 as patent reference 2).
With the methods described in patent references 1 and 2, a gloss difference formed between the leading and trailing edges of the recording medium may be solved. When, however, the recording medium to be employed is mainly of A6 size or B5 size with a length in the convey direction of 150 mm to 180 mm, in spite that the recording medium has a short length, since the process speed of the image printing apparatus is based on the sum of the standard size recording medium and the paper interval length as a reference, the printing productivity degrades greatly. In case of an A3-size recording medium, a gloss difference occurs at the boundary of an overlapping portion and a non-overlapping region, and in the worst case, an under-fixing error or the like occurs at the overlapping portion.